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The astronomers who comb through data on planetary surveys looking for potentially habitable planets are searching for worlds with certain specific characteristics. We assume that any planet capable of supporting life has to exist within the habitable zone (also sometimes called the “Goldilocks zone”) of its host star. Too close to the star and you bake, too far away and you’ll freeze. Liquid water is considered another key component required for life as we know it to exist. (Ammonia-based life has been theorized, but not yet demonstrated to be able to exist).

Astronomers now believe they’ve detected liquid water in the atmosphere of a planet orbiting within the habitable zone of its parent star. The planet, K2-18b (aka EPIC 201912552 b) orbits its red-dwarf star every 33 days. The red dwarf K2-18 is roughly 111 light-years from Earth and is ~2.7 percent as luminous as our own sun. The habitable zones of red dwarves are close to the star for this reason, and the authors note that the equilibrium temperature on K2-18b could be quite similar to Earth’s. The equilibrium temperature of a planet is the temperature it would have if it were a black body heated only by its star. The presence of an atmosphere and associated greenhouse effect means the actual temperature on the planet can be quite different from the equilibrium temperature.

The habitable zone of certain planets, Earth, and Mars for reference. K2-18b not shown above.

What the scientists are saying with this measurement is that Earth and this other planet start in roughly the same place, as far as their respective equilibrium temperatures are concerned (Earth is 257K, this K2-18b is 265K +/- 5K). The total amount of solar irradiation that Earth and K2-18b receive is approximately equal. The paper, which has not been peer-reviewed, notes that K2-18b is the least-massive planet to ever be detected with water vapor in its atmosphere.

“The water vapor detection was quite clear to us relatively early on,” lead author Björn Benneke, a professor at the Institute for Research on Exoplanets at the Université de Montréal, told Space.com in an interview. So he and his colleagues developed new analysis techniques to provide evidence that clouds made up of liquid water droplets likely exist on K2-18 b. “That’s in some ways the ‘holy grail’ of studying extrasolar planets … evidence of liquid water,” he said.

K2-12b probably isn’t habitable, at least not for humans. While the atmosphere contains water vapor, there’s evidence suggesting it’s quite thick, and that the planet may not have a surface in the first place.

While the study we’ve been discussing has not yet been peer-reviewed, Nature Astronomy has published the results of a different second study, which also confirms the presence of water vapor in K2-18b’s atmosphere. The lead scientist on the second study, Giovanna Tinetti, called these results “mind-blowing.”

“This is the first time that we have detected water on a planet in the habitable zone around a star where the temperature is potentially compatible with the presence of life,” she said.

We may not be flying off to explore K2-18b just yet — but now that we’ve found water vapor in the atmosphere of a planet orbiting inside a red dwarf’s habitable zone, we’re one step closer to finding a world that can work for creatures like ourselves. The James Webb Space Telescope, when it finally comes online, should be well-suited for exploring exoplanets like these.